FIG. 1 shows a ducted fan gas turbine engine 10 comprising in axial flow series: an air intake 12, a propulsive fan 14 having a plurality of fan blades 16, an intermediate pressure compressor 18, a high-pressure compressor 20, a combustor 22, a high-pressure turbine 24, an intermediate pressure turbine 26, a low-pressure turbine 28 and a core exhaust nozzle 30. A nacelle 32 generally surrounds the engine 10 and defines the intake 12, a bypass duct 34 and a bypass exhaust nozzle 36. The engine has a principal axis of rotation 31.
Air entering the intake 12 is accelerated by the fan 14 to produce a bypass flow and a core flow. The bypass flow travels down the bypass duct 34 and exits the bypass exhaust nozzle 36 to provide the majority of the propulsive thrust produced by the engine 10. The core flow enters in axial flow series the intermediate pressure compressor 18, high pressure compressor 20 and the combustor 22, where fuel is added to the compressed air and the mixture burnt. The hot combustion products expand through and drive the high, intermediate and low-pressure turbines 24, 26, 28 before being exhausted through the nozzle 30 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 24, 26, 28 respectively drive the high and intermediate pressure compressors 20, 18 and the fan 14 by concentric interconnecting shafts 38, 40, 42.
As will be appreciated, there is a need to compartmentalise the various sections of the engine so as to maintain the desired pressurised flow paths. The better this can be done, the more efficient the engine stands to be. However, the main gas path and inner core of the engine are made up from numerous parts which rotate relative to one another so sophisticated sealing technologies are required to seal between the relative rotating parts.
Conventional gas turbine engines employ many different types of seals at different locations throughout the engine. Such seal types include non-contacting, contacting, air riding or compliant seals.
A well utilised seal is a labyrinth seal. A labyrinth seal typically comprises a static part and a rotating part which are separated so as to be non-contacting in normal use. The rotating part includes a cascade of projecting annular fins which extend towards the static part. The opposing static part may include abradable portions which face the tips of the fins and preferentially abrade in favour of the fins if there is contact in use. Thus, the operating tolerance of the separating gap can be safely reduced to a minimum without fear of damaging the fins.
Labyrinth seals, as well as many other seals, exit a jet of air from the last fin in the cascade. The present invention seeks to provide an improved sealing arrangement.